The standard keyboard generally uses standard key codes defined with reference to ASCII and ISO646, wherein 128 symbols are expressed by 7 bits. The 128 symbols comprise 94 graphic symbols and 34 control characters. Therefore, the number of available key codes is limited and the expansion ability of hot key is influenced.
To overcome above problem, IBM had defined extended codes for special keys (such as Insert, Home, Page up, Page down) on keyboard wherein E0 code is prefixed to the standard key codes to broaden the function of keyboard.
However, the extended code proposed by IBM is applicable only to partial hot keys and partial standard keys and cannot be applicable to all keys on the keyboard. Moreover, the extended codes are permanently set and cannot be arbitrarily set or changed for temporary usage.